There is a product that is sold at every art supplies store that is used to glue paper together and it is "Clear" glue, usually water soluble. I always dab a little on the area to be decaled, spreading it evenly, to the point where it is a thin "Film", then place the decal on the glued surface and then do the usual pressure to remove as much moisture from under the decal possible.
The clear glue blocks/traps the air that is in the paint (as all matt paint has talcum powder in it that "stands up" creating the breakdown of the surface tension of the paint, eliminating any "shine") and the clear glue smothers these "effects" and lets the decal blend with the glue, eliminating the "silvering" effect totally.
The same result can be obtained by using wood glue, (PVA) but it will leave a "cloudy" residue if not done properly, and that is why I recommend the Clear paper glue.
The clear glue is exactly the same product as Humblol "Clear Fix".